The most realistic measure of cardiovascular function is the ability to respond to circulatory stress. Although stress tests have in past been restricted almost exclusively to physical exercise, there is increasing evidence that certain non-exercise stresses are important indicators of circulatory health; this is particularly true in the case of older individuals who are more vulnerable to environmental stresses. Although previous studies have indicated a progressively decreased response of older individuals to both exercise and non-exercise tests, the reasons for this decline are uncertain. Our understanding e.g. of the basic mechanisms involved in tolerance to such stresses or of the possible interrelationship between the response to physical exercise and to other circulatory stresses is inadequate. Yet our ability to alter circulatory performance in older people will depend first of all on a better understanding of these mechanisms and how they function in response to stress. In the present study, a first objective will be to compare the hemodynamic response of healthy men and women, age 20 to 85 years, to several carefully graded circulatory tests intended to simulate a variety of life stresses; these will include physical exercise, head-up tilt and lower body negative pressure (postural stress), the valsalva test (increased intrathoracic pressure) and local cold (thermal stress). The responses will be monitored primarily with non-invasive methods such as impedance cardiography and strain gauge plethysmography. A second objective will be to determine the effect of physical training on the non-exercise as well as exercise test performance of the different age groups and the cross tolerances (if any) among the test responses. Additional studies will be carried out to determine the effect of age on a) the carotid baroreflex and cardiopulmonary reflex responses, b) on peripheral vascular compliance and reactivity and c) on plasma catecholamine response to selected circulatory stresses. In the latter phases, emphasis will be placed on a study of the autonomic mechanisms involved.